1. Field of the Invention
The present invention relates to a radiation area extracting method for extracting a radiation area from image data. The present invention also relates to an image processing apparatus such as a radiography apparatus or digital camera.
2. Description of the Related Art
Recently, with the development of digital technology, a technique has been developed for converting radiation to image data as digital image signals, and performing image processing on the image data to display or print the output. In one method, radiation is received through a light receiver (also referred to as a light-receiving surface) and is stored on a stimulable phosphor sheet. Then, the stimulable phosphor sheet is illuminated with excitation light, and photostimulated luminescence from the stimulable phosphor sheet that is caused by this operation is photoelectrically detected to obtain image signals to produce a visible image. In another method, radiation is received through a light receiver that is a two-dimensional sensor and is directly converted to image signals. In yet another method, an analog film that has received radiation through a light receiver included in a cassette case is developed, and the image formed on the developed film is converted to image signals by an analog-to-digital (AD) conversion apparatus.
In radiography, in general, radiation focusing in which only an intended area is illuminated with radiation is performed to prevent radiation from affecting an area other than the intended area and from being scattered from the intended area so as to prevent a decrease in contrast. In this case, a first area that directly receives radiation and a second area that does not receive radiation other than secondary radiation such as scattered radiation are formed on a light receiver by the action of radiation focusing. The first area is referred to as a radiation area. Even when radiation focusing is not performed, radiation may not directly reach the light receiver. Even in this case, the area that directly receives radiation on the light receiver is referred to as a radiation area.
In general, in a case where parameters that are used when gray-level transformation is performed on image data are determined, the image data corresponding to the radiation area is used. Moreover, when image data is printed on a film, a range of the image data to be printed on the film is determined on the basis of the radiation area in some cases.
In this way, various types of processing are performed on the basis of the data of the radiation area in image data.
A method for extracting such a radiation area is disclosed in Japanese Patent No. 2596744. In this method, edge points that are supposed to constitute the edges of a radiation area are extracted in radial directions from the center of a stimulable phosphor sheet to the edges of the sheet. Then, assuming that candidate lines that represent the edges of a radiation area are lines that pass through a predetermined number or more of the extracted edge points, line segments that represent the edges of a final radiation area are extracted by the Hough transformation. Then, an area enclosed with these line segments is regarded as the final radiation area. In this method, the line segments that represent the edges of the final radiation area are not determined on the basis of the relationships between the extracted candidate lines.
Hitherto, line segments that represent the edges of a radiation area have been extracted on the basis of only the information of edge points. In this case, for example, when area segments other than the edges of the radiation area in the image data include straight parts, the radiation area is erroneously extracted in some cases. Thus, various types of solutions have been required.